Wanganui Formation.

No marine beds of this age are found in Otago, but probably some of the lacrustine series in the interior belong to this period. There are, however, considerable difficulties in classifying all our rocks and alluvial deposits that are of post-miocene age, as they form two connected series very complicated in their relations to one another, and it will require an immense amount of close observation, before each deposit can be put into its proper place.

For the present, I think it best to consider this formation in Otago to be confined to the old lake basins in the interior, forming the Maniototo Plains, Idaburn Valley, Manuherikia Valley, the low land lying between Cromwell and the Carrick Mountains, portions of the Ohau plain, Cardrona Valley, the basin at Gipps-town, the low land between Lake Hayes and Arrowtown, the Waipori basin, the margins of the Taieri Plain and Waihola Lake, the Tokomairiro Plains, and the low hills between them and Kaitangata Lake; and a consideable, but undefined, area in the Mararoa and the left bank of the Waiau, as far as Black Mount.

These deposits present considerable similarity in the different areas. The lowest bed in the interior basins appears, whenever it has been penetrated, to be fine compact clay; while round the margins of the basins the beds are cements and gravels, soft sandstone and sands, and clays and shales often associated with seams of lignite; the beds always dipping from the hills towards the interior of the basin.

At Hamilton, on the south-east side of the Maniototo plain, fine grey and purple clays, lying on soft sandstone, are seen dipping 62° N., (Fig 10,) the whole being covered, unconformable by pleistocene auriferous gravels.

Fig. 10.—Lacrustine deposits, Hamilton.
a. pleistocene gravels
b. grey clay Wanganui formation.
c. purple clay Wanganui formation.
d. soft sandstone Wanganui formation.

At St, Bathans, near the head of the Manuherikia Valley, quartz gravels with shales and lignite, dip at varying angles, sometimes reaching 35°S.W. These rest on decomposed schist, dipping 45° N.E., and are covered unconformably, by pleistocene quartz gravels, dipping 20° S. W., (Fig. 11). In this case, both the pleistocene gravels and the older ones belonging to the Wanganui formation, contain gold.

Fig 11.—Lacrustine deposits, St. Bathans,
a. Pleistocene gravels
b. Shales with lignite Wanganuiformation.
c. Gravels Wanganuiformation.
d. Schist (Kakanui formation).

In the bed of the Kawarau at Cromwell, sands and shales with lignite dip to the N.W. (fig. 12)., while further to the south, in the

Fig. 12. Lacrustine deposits, Cromwell:—a. Sand; b. Gravel (Pleistocene). c. Shale; d. Carbonaceous sand; e. Lignite; ƒ. Soft sandstone (Wanganui formation), g. Angular blocks of schist in gravel.

Bannockburn, soft sandstone and shales, with lignite, dip gently to the east, away from an isolated patch of schists, which stands out in the middle of the basin, and through which the Kawarau runs. On the north-western side of this hill, where the road goes after crossing the bridge, shales with leaves, probably belonging to this formation, dip 35° N.N.W.

Age.—Dr. Hector, who is the only geologist who has previously page 66written about these beds, refers them to the miocene period, considering them therefore to be anterior in date to the glacier period; but if my views are correct as to the glacier origin of the lakes, and also as to the date of the great glacier period, they must be of newer pliocene age. Dr. Hector also, in his section through Otago, published in the Quarterly Journal of the Geological Society, 1864, shews basalts breaking through and overlying lacrustine formations at Tuapeka and the Upper Taieri (Maniototo) plains; and in the section through the Manuherikia Valley that he sent to Professor Owen, which was published in the Trans. Zool. Society, V, he shews a "felspathic dyke" cutting through "older river gravels," or "older gold drift," of which the upper beds are "cemented with quartz." If these should be correct, they would considerably alter my views as to the age of the lacrustine beds, for we have no evidence of pleistocene volcanoes in Otago. I do not know the eruptive rocks mentioned by Dr. Hector at Tuapeka and the Manuherikia, but in the Maniototo Plains the lacrustine beds appear to me always to overlie the basalts.

The reasons that have induced me to refer these lacrustine deposits to the Wanganui formation will be given in the next section (Glacier period).

Fossils.—The only fossil as yet described from these beds is Unio aucklandica (?) from Clyde, but impressions of the same shell have been found in the Maniototo Plains and Millar’s Flat. Leaves also occur at Cromwell, and a deposit of diatomaceous earth occurs in Strath Taieri. This deposit is chiefly made up of three or four minute species of Cymbella or Cocconema, and although I have not been able to determine any of them, I have no doubt but that they are freshwater species.

In the Cardrona Valley, the lignite beds, according to Mr. Wright, are perpendicular, and interstratified with yellow clay and green sandstone. Where the Cardrona joins the Clutha, high cliffs are exposed of yellow clay capped with silt and rolled stones. The top of these cliffs appears to me to be higher than the moraine at the south end of Wanaka Lake. (Fig. 14, b, c.)

In the Upper Waiau Valley, the rocks in Redbank Creek are gravels and sands with semicarbonised wood. These gravels are irregularly stratified like river gravels, but they are highly inclined, and, indeed, in one place nearly vertical. The pebbles consist of sandstone, jasperoid slate, quartz, and conglomerate; I saw none of eruptive rocks nor of gneiss. They are smaller than the stones now brought down by the creeek, which would shew that the velocity of the creek was less, and that it probably fell into still water. These gravel beds are overlaid unconformably by younger gravels, the stones in which, like the creek bed at the present day, are composed chiefly of diorite, gneiss, and a conglomerate formed of small pebbles of jasperoid slate.